1. Field of the Invention
The present invention relates to an optical pickup actuator, and more particularly to an optical pickup actuator which can efficiently remove phase dispersion by a tolling mode of a lens holder caused by inconsistency between a weight center and a tracking force center of the lens holder.
2. Background of the Related Art
In general, an optical pickup actuator constantly maintains relative positions between an object lens and an optical recording medium (for example, disc), by moving constitutional elements (bobbin, lens holder, etc.) including the object lens. In addition, the optical pickup actuator records information and reproduces the recorded information, by following tracks of the optical recording medium.
FIGS. 1A, 1B and 2 are structure views illustrating a related optical pickup actuator.
Referring to FIGS. 1A, 1B and 2, the optical pickup actuator 100 includes a lens holder 102 having an object lens 101 at its center portion and being movable, and a magnetic circuit coupled to the sides of the lens holder 102, for moving the lens holder 102. The magnetic circuit includes tracking coils 106, focusing coils 105, yokes 103 and multipolar magnets 104.
The focusing coils 105 are adhered to the left and right sides of both sides of the lens holder 102 for focusing movement, facing vertical boundary surfaces 112 of polarities of the magnets 104, respectively. The tracking coils 106 are adhered to the centers of both sides of the lens holder 102 for tracking movement, facing horizontal boundary surfaces 113 of polarities of the magnets 104, respectively.
Still referring to FIG. 2, in the magnet 104, “”-shaped magnets 104a and 104b are disposed to be symmetrical to each other, and magnets 104c and 104d having the opposite polarities to polarities of the “”-shaped magnets 104a and 104b are disposed at the bottom left and right ends. Here, one multipolar magnet or four unipolar magnets can be used as the magnet 104.
The centers of the tracking coils 106 face the horizontal boundary surfaces 113 of the magnets 104a and 104b having different polarities, and the centers of the focusing coils 105 face the vertical boundary surfaces 112 of the magnets 104a, 104b, 104c and 104d having different polarities.
The magnets 104 are fixed to the inside surfaces of the yokes 103 which are ferromagnetic structures adjacent to the lens holder 102. The yokes 103 are coupled to a pickup base (not shown) by an integrating means.
Fixing units 108 are formed at both sides of the lens holder 102. One-side ends of two parallel wire suspensions 107 are fixed to each of the fixing units 108, and the other-side ends of the wire suspensions 107 are fixed to a circuit board 111 through a frame 109 formed on one side of the lens holder 102. The wire suspensions 107 serve as junction lines for lifting the lens holder 102 and supplying a current.
Here, a damper (not shown) is coupled to the inside of the frame 109 to give a damping characteristic to the wire suspensions 107 having rigidity. The other-side ends of the wire suspensions 107 are fixed to the circuit board 111 disposed outside the frame 109 by soldering.
The operation of the related optical pickup actuator 100 will now be explained. The focusing coils 105 adhered to the lens holder 102 are coiled in the horizontal direction When a current is supplied to the focusing coils 105, a magnetic flux is generated in the vertical direction. Here, a magnetic flux of the multipolar magnets 104 facing the focusing coils 105 is electromagnetically operated, to generate a force in the focusing coils 105 in the vertical direction. Accordingly, the lens holder 102 moves in the focusing direction (vertical up/down), to operate a focusing servo for compensating for a focusing error.
The tracking coils 106 adhered to the lens holder 102 are coiled in the vertical direction. When a current is supplied to the tracking coils 106, a magnetic flux is generated in the horizontal direction, and thus a repulsive force is generated by the fixed multipolar magnets 104 and the electromagnetic force. The lens holder 102 moves in the tracking direction (left, right) by the repulsive force, to operate a tracking servo for compensating for a tracking error.
As described above, the lens holder 102 moves in the tracking and focusing directions with the coils 105 and 106 adhered to its both sides, which is called a moving coil method. Conversely, multipolar magnets can be adhered to the outer circumference of the lens holder 102, and move with the lens holder 102, which is called a moving magnet method. The moving methods by the magnets and coils use the Lorentz's force of the Fleming's left hand law.
FIGS. 3 and 4 are structure views illustrating a related optical pickup actuator for radial tilting.
As illustrated in FIG. 3, radial tilting coils 217 are adhered to the circumferential surface of a lens holder 202, for moving the lens holder 202 in the radial tilting direction by an electromagnetic force with multipolar magnets 204.
That is, as shown in FIG. 4, when a current is supplied to the radial tilting coils 217 coiled on the circumferential surface of the lens holder 202, the left and right sides of the lens holder 202 are rotated in the opposite directions by different polarities of the multipolar magnets 204a and 204b facing the radial tilting coils 217.
In FIG. 3, reference numeral 201 denotes an object lens, 203 denotes a yoke, 205 denotes a focusing coil, 206 denotes a tracking coil, 207 denotes a wire suspension, and 209 denotes a frame.
The optical pickup actuator performs motion in a movable coil method by a magnetic field of the permanent magnet, and moves the object lens to a target position of an optical recording medium. Here, the lens holder which is a moving part of the optical pickup actuator is fixed by the wire suspensions having rigidity and damping characteristic, thereby obtaining a target frequency characteristic. In addition, the lens holder performs translation in the focusing direction and the tracking direction which are vertical to each other. In order to reduce an error of an optical signal, the lens holder must perform motion without unnecessary vibrations such as rotation or twisting.
However, as depicted in FIG. 5, when the lens holder is driven in the tracking direction, a weight center WC and a tracking force center TC are inconsistent. Therefore, the lens holder is operated in a rolling mode in a high band frequency, and phase dispersion is caused in the rolling frequency. Generally, the rolling frequency is generated between 100 and 120 Hz, may be identical to a rotary frequency of a disc, and has detrimental effects on a control system due to a tilted angle.
FIG. 6 is a graph showing the rolling mode effects of the related optical pickup actuator through relations between a gain (DB) and a phase (degree) by frequency. The rolling mode is generated in about 100 Hz. That is, rolling is caused by inconsistency between the force centers TC and WC in the high band.
Still referring to FIG. 5, rolling is caused by inconsistency between the force center WC in the height direction and the force center TC in the tracking direction. That is, the force center in the height direction is disposed at the center of the lens holder, but the force center in the tracking direction is disposed at the center of the tracking coil. When the lens holder is driven in the tracking direction, the force center in the tracking direction is changed due to unbalance of the magnetic force generated between the magnets and the tracking coils.
In addition, the force center is upwardly inclined from the very center due to magnetic flux distribution by the multipolar magnets. To solve the above problems, as shown in FIGS. 1A and 3, dummy masses 120 and 220 are adhered to the top ends of the lens holder, for lifting the weight center WC. However, the dummy masses 120 and 220 increase a weight of the lens holder, to reduce sensitivity in the high band.